Adrenal insufficiency is frequently associated with an impaired ability to concentrate or dilute the urine. Recent evidence indicates that reduced solute delivery to the thick ascending limb and persistent vasopressin secretion are important factors in producing these defects, but other evidence indicates that additional factors are involved. Some evidence for effects on solute transport by the thick ascending limb, vasopressin responsivenss of the collecting tubule, and, perhaps, urea handling by the kidney hasbeen obtained, but the evidence is indirect or obtained in amphibian, rather than mammalian, tissue. Convincing evidence for these possibilities will not be obtained with clearance methods because the data are indirect and subject to multiple interpretations. Micropuncture techniques are also not satisfactory because the appropriate segments of the nephron are deep in the kidney. I propose to examine these questions directly by obtaining fragments of the appropriate segments of the nephron segments and studying their functional characteristics in vitro. This methodologic approach will exclude factors such as solute delivery, changes in blood flow, glomerular filtration rate, medullary solute concentration, and many other important variables. The effect of adrenalectomy and steroid replacement will be examined in the thick ascending limb and the cortical collecting tubule. Urea permeability will be studied in these and other segments of the nephron. Physiologic concentrations of steroid hormones will be used to separate mineralocorticoid from glucocorticoid effects. The mechanism of the steroid effects will also be examined. Preliminary evidence suggests that the use of mouse tubules may be superior to the usual rabbit tubules for the study of vasopressin effects. Although not a primary objective, this observation is of significance for the present study and for future studies of vasopressin effects in vitro. Some time will therefore be spent in developing this model.